Among display devices such as liquid crystal devices, there have been developed display devices with a touchscreen panel which has a touchscreen panel (coordinate sensor) function designed such that when a finger or an input pen touches the surface of the panel, the touched position can be detected.
Conventionally, such display devices with a touchscreen panel have been mainly display devices using a so-called a resistive touchscreen panel or a capacitive touchscreen panel.
However, such display devices require, for example, a special panel for detecting a position, resulting in a problem that the device as a whole becomes thicker. Furthermore, providing such a touchscreen panel on a screen (display region) of the display device raises a problem that visibility drops.
For this reason, recently, instead of display devices including a resistive touchscreen panel or capacitive touchscreen panel, there has been developed a touchscreen panel-integrated display device in which so-called two-dimensional sensor array is built, which includes light receiving elements (optical sensor elements) such as photodiodes and phototransistors positioned in a matrix manner.
However, since such a display device includes light receiving elements in a screen, there arises a problem that an open area ratio drops. In addition, there arises a problem that an optical signal reading circuit gets complicated.
Furthermore, in a case where bus lines (scanning signal lines and display data signal lines) of display elements (driving elements) such as TFTs (Thin Film Transistors) double as bus lines (scanning signal lines and data read lines) of light receiving elements so that display and sensing are performed in a time-dividing manner in order to avoid the drop in the open area ratio, operation speed is limited.
In order to deal with these problems, there has been developed a touchscreen panel-integrated display device employing optical scanning. Such a display device is free from problems such as the drop in the open area ratio and limitation of operation speed.
The touchscreen panel-integrated display device employing optical scanning is a display device designed such that light scans a panel surface and detects blocking of the light by a finger etc. to detect the position of the finger.
For example, Patent Literature 1 describes a touchscreen panel-integrated display device employing optical scanning.
FIG. 11 is a drawing schematically showing a whole configuration of the display device.
As shown in FIG. 11, outside both ends of a short side of a rectangular display screen 100 (right side in the drawing) to be touched by an indicating object (recognized object) S which is a finger, a pen etc., there are provided light transmitting and receiving units 101a and 101b having therein optical systems including light emitting elements 111a and 111b, light receiving elements 113a and 113b, and polygon mirrors 116a and 116b etc., respectively. Outside three sides other than the right side of the display screen 100, there is provided a recursive reflective sheet 102.
The following explains scanning by light projected from the light transmitting and receiving unit 101b. Light projected from the light transmitting and receiving unit 101b scans counterclockwise in FIG. 11 from a scan start position at which the light is directly incident to the light receiving element 113b to a position at which the light is blocked by a light blocking member 170 for preventing the light from the light transmitting and receiving unit 101b from being incident to the light transmitting and receiving unit 101a, and to a position (Ps) at which the light is reflected by an end of the recursive reflective sheet 102. Thereafter, the projected light is reflected by the recursive reflective sheet 102 until a position (P1) at which the light reaches one end of the indicating object S. Subsequently, until a position (P2) at which the projected light reaches the other end of the indicating object S, the light is blocked by the indicating object S, and from the position (P2) to a scan position (Pe), the projected light is reflected by the recursive reflective sheet 102.
The light receiving element 113b detects reflective light from the recursive reflective sheet 102, and when the level of the received reflective light is smaller than a predetermined threshold at a certain area, the light receiving element 113b identifies the area as a blocked area where light is blocked by a finger or a pen.
In the display device described in Patent Literature 1, when the light receiving element 113b judges that there is no indicating object S, light emission by the light emitting elements 111a and 111b is stopped. While the light emission is stopped, a predetermined margin voltage is added to values detected by the light receiving elements 113a and 113b to obtain the threshold.
The margin voltage is determined based on fluctuation in the amount of received light accompanying noises in a light receiving system, digitalization error in A/D conversion, accumulated data of received light in a time line etc.